Bonding process with inhibited oxide formation

ABSTRACT

First and second contacts are formed on first and second wafers from disparate first and second conductive materials, at least one of which is subject to surface oxidation when exposed to air. A layer of oxide-inhibiting material is disposed over a bonding surface of the first contact and the first and second wafers are positioned relative to one another such that a bonding surface of the second contact is in physical contact with the layer of oxide-inhibiting material. Thereafter, the first and second contacts and the layer of oxide-inhibiting material are heated to a temperature that renders the first and second contacts and the layer of oxide-inhibiting material to liquid phases such that at least the first and second contacts alloy into a eutectic bond.

PRIORITY, CROSS-REFERENCES, INCORPORATION BY REFERENCE

This application is a divisional of U.S. patent application Ser. No.17/138,255, filed Dec. 30, 2020, which is a divisional of U.S. patentapplication Ser. No. 16/702,783, filed Dec. 4, 2019 (now U.S. patentSer. No. 10/910,341), which is a divisional of U.S. patent applicationSer. No. 16/222,939 filed Dec. 17, 2018 (now U.S. patent Ser. No.10/541,224), which is a divisional of U.S. patent application Ser. No.15/709,371 filed Sep. 19, 2017 (now U.S. patent Ser. No. 10/192,850),which claims priority to U.S. Provisional Patent Application No.62/396,817 filed Sep. 19, 2016. Each of the above-identifiedapplications is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to semiconductor wafer-to-wafer,die-to-wafer and die-to-die bonding.

BACKGROUND

In a solder system containing aluminum on one side, as in the case of aCMOS wafer, the problem exists of rapid native oxidation of the aluminumsurface—forming an oxide layer that can impede solder bonding generally,and wafer-to-wafer bonding in particular. Common methods used to combatthe oxide formation include pre-bond cleaning (plasma, chemical), highforce during bonding (breaking the oxide), and gas treatment prior tobonding (forming gas at temperature), all of which add complexity andcost to the bonding operation.

DRAWINGS

The various embodiments disclosed herein are illustrated by way ofexample, and not by way of limitation, in the figures of theaccompanying drawings and in which like reference numerals refer tosimilar elements and in which:

FIG. 1 contrasts a conventional, oxide-plagued bonding approach with anexemplary oxide-inhibited bonding technique;

FIG. 2 illustrates a variety of materials that may be used to enableoxide-inhibited bonding of counterpart substrates;

FIG. 3 illustrates the same bonding materials as FIG. 2 , but withoxide-inhibitant disposed on the bonding surfaces on both counterpartsubstrates; and

FIG. 4 illustrates the optional bonding materials of FIG. 2 (Al, AlSi,AlSiCu) disposed on counterpart substrates and capped with silver oxideinhibitant rather than the gold oxide inhibitant shown in prior drawingfigures.

DETAILED DESCRIPTION

In various embodiments herein, bonding surface(s) of conductive contactsdeposited or otherwise formed on a wafer or die is capped with anothermaterial to inhibit (or prevent, limit or control) the oxidation of thebonding surface. By using a capping material that does not oxidize orhas an easier to remove oxide, steps commonly used to remove undesiredoxide can be reduced or eliminated from the bonding process. Inparticular, bonding can be performed at substantially lower force whichadvantageously lessens relative movement of the precisely-alignedsubstrates during wafer bonding, flowing of the solder during liquidus,etc.

Materials that may be used for capping the aluminum layer include, forexample and without limitation, the family of noble materials (e.g.,rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium,platinum, gold, etc. and various alloys thereof) and other materialssuch as copper, titanium, nickel, indium, tin, and zinc. Materials thatform some oxide, but are softer than aluminum are also useful as lowerforce can be used to cause mechanical deformation and thereby exposeun-oxidized material at the bonding surface of the contact.

In one embodiment, the capping material is deposited on the aluminum inan environment that is free of oxidation (e.g., in the same chamber, ormultiple deposition chambers connected by a common vacuum chamber), orin a chemical environment such as a plating bath, etc. in whichoxidation is removed as part of the process. Where oxide removal is notinherent in the inhibitant disposition process, any exposure of thebonding surface to an oxidizing environment is limited in time and/orconcentration (i.e., limited concentration of oxidizing agents) suchthat the exposed contact surface remains primarily that of the bondingsurface material (e.g., aluminum, aluminum alloy, etc.) In yet otherembodiments, the substrate with aluminum or other oxidation-pronebonding surface is further processed, and then later cleaned (forexample in a sputter etch) to remove the oxide before depositing thecapping material (again, with limited pre-capping exposure to anoxidizing environment) to form a final structure with a layer ofaluminum (including any of various aluminum alloys) and the cappingmaterial.

In practice, the capping material is chosen for compatibility with theoverall requirements of the intended solder bond. For example, in thecase of a binary eutectic (two materials forming a eutectic bond), thecapping material may form either with the aluminum alloy, or with thecomplementary substrate material, or both. The resulting ternary system(or quaternary, or higher material count system) is generally chosen tomelt at a reasonable/tolerable temperature in view of the systemelements, and to form a stable solder joint.

FIG. 1 contrasts a conventional, oxide-plagued bonding approach(relatively high force applied to break aluminum oxide) with anexemplary oxide-inhibited bonding technique in which aluminum contactsare capped with thin gold films to inhibit oxide formation and therebyenable formation of eutectic wafer-to-wafer (or die-to-wafer ordie-to-die) bonds with substantially reduced applied force.

FIG. 2 illustrates a variety of materials that may be used to enableoxide-inhibited bonding of counterpart substrates 121 and 123. As shown,aluminum (optionally alloyed with trace to significant amounts ofsilicon and/or copper, for example) is disposed on substrate 121 andcapped with an oxide inhibitant of Gold (e.g., 10-100 nm thick, thoughthicker or thinner films may be used), while silicon, germanium, or analloy of both (SiGe) is disposed on the counterpart substrate 123. Anyof the bonding materials on substrate 121 may be bonded with any of thebonding materials on substrate 123 to form a bond within a given system,and a diversity of bond types may exist within the same system.Substrates 121 and 123 may themselves be different wafers (e.g., twodifferent CMOS wafers, a CMOS wafer and a microelectromechanical-system(MEMS) wafer, etc. to be singulated into individual dies afterwafer-to-wafer bonding), different dies, or a die and a wafer. Withrespect to different wafer or die types (e.g., MEMS and CMOS), any ofthe bonding material pairs may be reversed in orientation from thosedepicted (e.g., Si/Ge/SiGe may be disposed on MEMS wafer or CMOS wafer)and a diversity of such orientations may exist within the same system.Also, either or both of substrates 121 and 123 may individually includeone or more constituent substrates and/or other structural components(e.g., resonant MEMS structures, metal layers, oxide layers, vias,encapsulation chambers, etc.).

FIG. 3 illustrates the same bonding materials as FIG. 2 , but withoxide-inhibitant (gold in the examples of FIGS. 2 and 3 ) disposed onthe bonding surfaces on each of substrates 131 and 133. Thus, aluminum(or alloy) with a cap of gold is disposed on substrate 131, andsilicon/germanium (one, the other, or alloy of the two) with a cap ofgold is disposed on substrate 133. As in FIG. 3 , any of the bondingmaterials on substrate 131 may be bonded with any of the bondingmaterials on substrate 133 to form a bond within a given system, and adiversity of bond types may exist within the same system. Also,substrates 131 and 133 may be different wafers (e.g., two different CMOSwafers, a CMOS wafer and a microelectromechanical-system (MEMS) wafer,etc. to be singulated into individual dies after wafer-to-waferbonding), different dies, or a die and a wafer, and either or both ofsubstrates 131 and 133 may individually include one or more constituentsubstrates and/or other structural components (e.g., resonant MEMSstructures, vias, encapsulation chambers, etc.). With respect todifferent wafer or die types (e.g., MEMS and CMOS), any of the bondingmaterial pairs may be reversed in orientation from those depicted (e.g.,Si/Ge/SiGe may be disposed on MEMS wafer or CMOS wafer) and a diversityof such orientations may exist within the same system.

Despite the gold-cap oxide inhibitant shown in FIGS. 2 and 3 , any ofthe depicted bonding surfaces/materials may be capped with oxideinhibitants other than gold. FIG. 4 , for example, illustrates thebonding materials of FIG. 2 (Al, AlSi, AlSiCu) disposed on counterpartsubstrates 141 and 143 and capped with oxide inhibitants of silverinstead of gold. The counterpart bonding surfaces may similarly becapped with silver instead of gold (i.e., arrangement shown in FIG. 3 ,but with silver inhibitant instead of gold).

In general, oxide-inhibited bonding processes according to thetechniques shown and described herein involve substrate alignment (e.g.,wafer alignment in a wafer bond, singulated die alignment in adie-to-die bond), substrate-to-substrate contact with varying degrees offorce (including a force ramp), and then elevation (e.g., ramp) to atleast a first temperature where particular binary combinations reachliquidus (for example, a temperature at which silicon with gold capreaches liquidus). Depending on the choice of materials, the ternary orlarger combination may reach liquidus upon elevation to the firsttemperature, or, if not, further elevation to a second temperature andpossibly additional elevations to third, or higher temperature targetsare carried out to achieve liquidus of the ternary (or quaternary, etc.)system. In embodiments having multiple different liquidus temperatures,elevation to each temperature may be accompanied by a pause of variableand/or controlled duration (i.e., plateau at a particular temperature)before commencing further elevation toward the higher temperature(plateau). In an alternative embodiment having multiple liquidustemperatures, the temperature may be raised directly to a higher thaneutectic temperature which might be useful in achieving certain alloycompositions. In another embodiment, prior to substrate alignment andbonding, one or both wafers are heated to one or more predeterminedtemperatures to alloy the oxide-inhibiting material with the conductivematerial that constitutes the underlying contact. In general, heating toa single alloy-forming temperature (“alloying temperature”) issufficient where oxide-inhibitant is disposed over the bonding surfacesof only one of the counterpart wafers or where a single temperaturesetpoint is sufficient to alloy respective dispositions ofoxide-inhibitant and underlying contacts on both of counterpart wafers.Conversely, where alloying temperatures of oxide-inhibitant andunderlying contacts are substantially different with respect tocounterpart wafers, each wafer may be separately heated to a respectivealloying temperature. Any or all of the alloying temperatures may behigher or lower than the eutectic bonding temperature. Also, in allcases, temperature elevation for alloying or bonding purposes may bemonotonic (until eventual cool down) or may be characterized by one ormore valleys or inflections.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding of the disclosed embodiments. In some instances,the terminology and symbols may imply specific details that are notrequired to practice those embodiments. The term “contact” hereingenerally refers to a conductive material that makes up part of aconductive bond, though “physical contact” refers to physical touching—adistinction generally clear from context. “Contact interface” refers toa bond interface. The terms “exemplary” and “embodiment” are used toexpress an example, not a preference or requirement. Also, the terms“may” and “can” are used interchangeably to denote optional(permissible) subject matter. The absence of either term should not beconstrued as meaning that a given feature or technique is required.

Various modifications and changes can be made to the embodimentspresented herein without departing from the broader spirit and scope ofthe disclosure. For example, features or aspects of any of theembodiments can be applied in combination with any other of theembodiments or in place of counterpart features or aspects thereof.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. An apparatus comprising: a first die having afirst electrical contact; and a second die having a second electricalcontact; wherein the apparatus is fabricated according to a process inwhich an oxide-inhibiting material, disposed on a surface of one of thefirst electrical contact and the second electrical contact, is meltedand forms a bond region between the first electrical contact and thesecond electrical contact, the bond region having a materialconstituency that is at least in part different than either the firstelectrical contact or the second electrical contact; wherein: the firstelectrical contact comprises one of aluminum, a mixture ofaluminum-silicon, a mixture of aluminum-copper and a mixture ofaluminum-silicon-copper; and the second electrical contact comprises oneof silicon, germanium, and a mixture of silicon-germanium.
 2. Theapparatus of claim 1 wherein the apparatus is fabricated according to aprocess in which the oxide-inhibiting material is disposed so as to capa conductive surface of the first electrical contact, according to aprocess in which the first die is positioned so as to place theoxide-inhibiting material between the conductive surface of the firstelectrical contact and a conductive surface of the second electricalcontact and according to a process in which the oxide-inhibitingmaterial is then melted to form the bond region.
 3. The apparatus ofclaim 1 wherein the oxide-inhibiting material comprises a capping layerof no more than 100 nanometers thickness prior to melting of theoxide-inhibiting material.
 4. The apparatus of claim 1 wherein the bondregion comprises a eutectic bond.
 5. The apparatus of claim 1 whereinthe oxide-inhibiting material comprises at least one of a noblematerial, copper, titanium, nickel, iridium, tin and zinc.
 6. Theapparatus of claim 1 wherein the oxide-inhibiting material comprises atleast one of rhenium, ruthenium, rhodium, palladium, silver, osmium,iridium, platinum, gold and an alloy thereof.
 7. The apparatus of claim1 wherein the oxide-inhibiting material comprises at least one of goldand silver.
 8. The apparatus of claim 1 wherein the apparatus isfabricated according to a process in which a first wafer bearing thefirst die is aligned with a second wafer bearing the second die, and inwhich apparatus is cut from the first die and the second die oncebonded.
 9. The apparatus of claim 1 wherein the apparatus is fabricatedaccording to a process in which force is applied to urge the firstelectrical contact toward the second electrical contact in connectionwith melting of the oxide-inhibiting material.
 10. The apparatus ofclaim 1 wherein the bond region comprises a mixture of material from theoxide inhibiting material and material from at least one of the firstelectrical contact and the second electrical contact.
 11. An apparatuscomprising: a first die having a first electrical contact; and a seconddie having a second electrical contact; wherein the apparatus isfabricated according to a process in which an oxide-inhibiting material,disposed on a surface of one of the first electrical contact and thesecond electrical contact, is melted and forms a bond region between thefirst electrical contact and the second electrical contact, the bondregion having a material constituency that is at least in part differentthan either the first electrical contact or the second electricalcontact; wherein the first electrical contact comprises one of aluminum,a mixture of aluminum-silicon, a mixture of aluminum-copper and amixture of aluminum-silicon-copper; wherein the second electricalcontact comprises one of silicon, germanium, and a mixture ofsilicon-germanium; and wherein a first one of the first die and thesecond die comprises a microelectromechanical system (MEMS) structureand a second one of the first die and the second die comprises acomplementary metal-oxide-semiconductor (CMOS) structure.
 12. Theapparatus of claim 11 wherein the first electrical contact comprisesaluminum and wherein the first die comprises the CMOS structure.
 13. Theapparatus of claim 11 wherein the apparatus is fabricated according to aprocess in which the oxide-inhibiting material is disposed so as to capa conductive surface of the first electrical contact, according to aprocess in which the first die is positioned so as to place theoxide-inhibiting material between the conductive surface of the firstelectrical contact and a conductive surface of the second electricalcontact and according to a process in which the oxide-inhibitingmaterial is then melted to form the bond region.
 14. The apparatus ofclaim 11 wherein the oxide-inhibiting material comprises a capping layerof no more than 100 nanometers thickness prior to melting of theoxide-inhibiting material.
 15. The apparatus of claim 14 wherein thebond region comprises a eutectic bond, and wherein the capping layercomprises at least one of rhenium, ruthenium, rhodium, palladium,silver, osmium, iridium, platinum, gold and an alloy thereof.
 16. Theapparatus of claim 14 wherein the capping layer comprises at least oneof gold and silver.
 17. The apparatus of claim 11 wherein the apparatusis fabricated according to a process in which a first wafer bearing thefirst die is aligned with a second wafer bearing the second die, and inwhich apparatus is cut from the first die and the second die oncebonded.
 18. The apparatus of claim 11 wherein the apparatus isfabricated according to a process in which force is applied to urge thefirst electrical contact toward the second electrical contact inconnection with melting of the oxide-inhibiting material.
 19. Theapparatus of claim 11 wherein the bond region comprises a mixture ofmaterial from the oxide inhibiting material and material from at leastone of the first electrical contact and the second electrical contact.20. An apparatus comprising: a complementary metal-oxide-semiconductor(CMOS) die having a first electrical contact; and amicroelectromechanical systems (MEMS) die having a second electricalcontact; wherein the first electrical contact comprises one of aluminum,a mixture of aluminum-silicon, a mixture of aluminum-copper, and amixture of aluminum-silicon-copper; and wherein the second electricalcontact comprises one of silicon, germanium, and a mixture ofsilicon-germanium; and wherein the apparatus is fabricated according toa process in which an oxide-inhibiting material disposed on a surface ofone of the first electrical contact and the second electrical contact ismelted to form a bond region between the first electrical contact andthe second electrical contact, the bond region having a materialconstituency that is at least in part different than either the firstelectrical contact or the second electrical contact.
 21. The apparatusof claim 20 wherein the bond region corresponds to a eutectic bond andwherein the oxide-inhibiting material comprises at least one of rhenium,ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, goldand an alloy thereof.
 22. The apparatus of claim 20 wherein theapparatus is fabricated according to a process in which a first waferbearing the first die is aligned with a second wafer bearing the seconddie, and in which apparatus is cut from the first die and the second dieonce bonded.